Allowable bearing pressures for wall and columns 1

[Okiryu]

Hi, I have a 2-story structure with high column loads (D+L=1,050kN). The wall loads are around 90kN/m. The structure will be like a box (rectangular-shaped building with continous footings for walls and independent footings for columns)

I have CL and CH soils for this site and am planning to provide 2 allowable bearing pressures: one for the columns (independent footings) and other for the walls (continuos footings). My intent is to provide higher allowable bearing pressures for the walls than the columns in order to keep differential settlements lesser than the typical 12 mm. I do not know the column spacing so I have not calculated the deflection ratios (delta/L) yet.

My question is: do you think that providing 2 allowable bearing pressures for the same structure is acceptable? Do you think that this can cause confusion to the structural designer?

 

[hokie66]

It is common practice to design column and wall footings with different bearing pressures, for the reason you stated. This should not confuse a structural engineer.

 

[Okiryu]

hokie66, thanks for the confirmation... it is good to hear that especially if it comes from a structural engineer.

 

[Mercycity]

I have a problem with CL and CH soils if is okay to support type of struture without soil improvement. Such type of soil can undergo consolidation settlement and destroy the structure. Look possibilty of using deep foundation or raft foundation.

 

[Okiryu]

That would not happen if you provide adequate allowable bearing pressures in order to settlements be within tolerable limits.

 

[oldestguy]

Hey fellas there is another factor for the CH soils. That's volume change with change of moisture content. Generally the most trouble I have seen is with fast growing trees nearby drawing water from the soil and causing considerable differential settlement. It happens regardless of your footing design pressure. There have been other factors, such as not well thought out (or stupid) drain systems. Designing and constructing to avoid these troubles can be quite complicated.

 

[emmgjld]

Regional climate, landscape irrigation practices Geology & the actual soils (geotechnical) conditions AND Regional Expectation/Habits control. See oldestguy here and other posts. I have followed what my father established in the Colorado Springs area in the late 50's & 60's. Please note that He did & I still design at least 1/2 to 3/4 of the foundations which I/we provided geotechnical data for.

What I have done in my arid to semi-arid, heavily irrigated area for small to medium loads is specify a maximum allowable bearing for the structure foundation, call for the interior footings to be balanced for about 300 to 500 psf less for settlement issues (500 psf more for expansive issues). The other requirement is for the foundation to be balanced within about 200-400 psf for small structures, after satisfying the interior versus exterior requirements. Larger structures are have about the same balancing requirements, at my discretion.

I must add that these requirements change in different areas of my practice & may radically change for certain Architects & Structural Engineers. The concept of good foundation balancing is not appreciated by many.

 

[Okiryu]

Thank you for your comments. Yes, CH soils can be problematic if proper care is not taken. Although I add some caution notes about considerations for good drainage systems in my reports, based on OG comments, I should start thinking on add some recommendations about avoiding trees nearby the structures. I have seen these comments from OG in past threads, but it is a good reminder. Also, I think that results from the subsurface investigations and the project requirements are other factors that will dictate the foundation options which need to be designed safely and economically.

 

[hokie66]

Just a caveat...all CH soils are not the same relative to shrink-swell behaviour.

 

[BigH]

There is no question that you can use different allowable bearing pressures. To provide the same settlement for your various footings, you can develop a chart of allowable bearing pressure vs width of footing. Note that for the spread footing, you would find the depth of influence to be in the order of 2 times footing width; for wall footings, 4 times footing width. If you have footings that might overlap, you can do charts, too, of spread difference between footings. All this for estimating the same settlement for your various footing scenarios. Of course, remember, please, that estimates of settlements - especially for clays is an estimate - and if you are within 30% of the actual settlement, you've done well.

Attached is a rough start on such a graph I made years and years and years ago - it just happened to have been following me around Asia for the last 20 odd years. Obviously, this is made for a particular set of strata and undrained shear strengths.

 

[Okiryu]

BigH, thanks for chart, it is good reference. I assume that for this chart, the maximum size of footings for a max. settlement of 1 in. is 20 feet. Footings larger than 20 ft, will need to be designed to an allowable bearing pressure as shown in dotted line to keep settlements lesser than 1 in. So, basically, just the top portion of the graph (allow. bearing pressures vs. size of footings) may be needed in the reports?

 

[BigH]

We used to include the full chart - on that particular chart - for footings larger than 20 ft, the dotted line showed the decrease in allowable bearing pressure to maintain a maximum of 1 inch. You can develop similar charts for, say, 40 mm (as they use in India) - or other maximum settlements - I think I've seen a similar chart in WSDOT's Geotechnical Manual

 

[Okiryu]

Thanks BigH, I have not seen reports which include this type of graphs. But they are very useful, specially when the loads we receive from the structural are preliminary, so this type of charts will help if loads change during the design process.